1. Technical Field
This invention relates to radio spectrum blending. In particular, this invention relates to high definition (HD) radio blending.
2. Related Art
Digital information may be transmitted using frequency division multiplexing (FDM). FDM is a modulation method that has been used in a number of different digital television and radio systems, including digital video broadcasting for television (DVB-T). In a hybrid mode of high definition (HD) radio, the amplitude modulation (AM) version may carry 36 kilobits per second of data for the main audio channel, while frequency modulation (FM) stations may carry information at 96 kbit/s. HD radio may also be used to carry multiple distinct audio services, called multicasting. Secondary channels, such as for weather, traffic, or a radio reading service, may be added this way, though it may reduce the audio quality of all channels on a station. Datacasting is also possible, and radio data system (RDS)-like metadata about the program and station may be included in the standard signal.
A limitation of HD radio is the blending process. In hybrid mode, a radio may lock onto an analog signal first, then try to find a better quality digital signal. If the digital signal is lost, the receiver may blend to analog. Much of the success of this blending relies on proper synchronization of the analog and digital audio signals by the broadcaster at the transmitter. This fallback process may also be impeded by the use of multiple channels.
Another limitation of conventional HD radio blending is the switching back and forth between an analog and a digital signal, which may result in sudden loudness changes perceived by the listener. If the listener is passing through a region of varying digital signal strength, this change may occur often, resulting in a decreased audio experience. For an AM HD radio signal, the blend from analog to digital may also result in an expansion of frequency content in the received signal. This change in frequency response may result in an uneven quality signal perceived by the listener.
Yet another limitation involves switching between a digital signal and an analog signal when the digital and analog signal are uncorrelated in time. Many stations switch off a delay in the analog signal during ball games. A user may perceive skipping over portions of audio content, or a repeat of audio content when the digital and analog components are uncorrelated. Therefore a need exists for an HD radio blending algorithm that reduces analog-digital switching drawbacks.